1. Technical Field
The present invention relates to a solid electrolyte, a carbon dioxide sensor for use in indoor or outdoor environmental control, an agricultural process of protected horticulture, prevention against disasters, measurement of an organism surface for a metabolic function, and the like, and a correction method thereof.
2. Technical Background
In recent years, the need for a carbon dioxide sensor is increasing mainly for the detection of contamination of indoor air with widening use of air conditioning, the detection of contamination of air in livestock production facility, the control of plant growth in protected horticulture and various industrial processes, and carbon dioxide sensors according to a variety of methods have been proposed.
Specifically, for example, a carbon dioxide sensor for which an infrared absorption method is applied is practically used. However, the sensor according to the above method has not yet come into wide use since it is large in size and expensive. Further, a semiconductor-applied sensor has been proposed, while the measurement of a carbon dioxide concentration alone is difficult since the sensor has poor selectivity to carbon dioxide.
Meanwhile, several sensors using a solid electrolyte have been proposed as small-sized and inexpensive sensors. Of these sensors, the simplest sensor is a concentration polarization sensor having a pair of electrodes formed on a solid electrolyte such as potassium carbonate, which forms a dissociation equilibrium with carbon dioxide, and the sensor measures an electromotive force based on a concentration difference of ambient gas by bringing a reference gas having a known concentration into contact with one electrode. The problem of the above sensor is that a high temperature of 500 to 700.degree. C. is required for imparting the solid electrolyte with a necessary conductivity and that the solid electrolyte has a property that its material is extremely susceptible to a humidity. The above high temperature can be attained by providing a built-in heater in the sensor. In this case, however, a convection current and a change in temperature are caused in an ambient atmosphere so that an external environment is affected, which is undesirable in some fields of use. There is another problem that the power consumption increases.
In addition to the above concentration polarization sensor, there is a so-called an electromotive force detection sensor obtained by forming a pair of electrodes on a solid electrolyte having an alkali metal ion conductivity such as NASICON (sodium super ion conductor: Na.sub.3 Zr.sub.2 Si.sub.2 PO.sub.12), forming a layer of metal carbonate such as sodium carbonate which forms a dissociation equilibrium with carbon dioxide on one electrode to be used as a detection electrode, and forming the other electrode as a carbon dioxide non-sensitive electrode. This sensor detects carbon dioxide by utilizing a kind of an electric cell, and the electric cell expression is as follows.
CO.sub.2, O.sub.2, Pt.vertline.Na.sub.2 CO.sub.3.vertline..vertline.NASICON.vertline.Pt,O.sub.2 PA1 wherein E.sub.0 is a constant, R is a gas constant, T is an absolute temperature, F is Faraday constant, 2Na.sub.2 O is an activity of Na.sub.2 O, and PCO.sub.2 is a partial pressure of carbon dioxide gas. PA1 the detection electrode containing a metal carbonate which forms a dissociation equilibrium with carbon dioxide, PA1 the solid electrolyte being a product formed by heat-treating a compound having an ammonium ion portion and a metal halide. PA1 further having a temperature detection element for detecting a temperature of the carbon dioxide sensor and/or a humidity detection element for detecting a humidity around the carbon dioxide sensor. PA1 measuring an electromotive force with the carbon dioxide sensor and at the same time detecting a temperature with the temperature detection element, PA1 correcting the electromotive force with a linear relational expression of the electromotive force and the temperature, and PA1 determining a carbon dioxide concentration on the basis of the corrected electromotive force. PA1 measuring an electromotive force with the carbon dioxide sensor and at the same time detecting a humidity with the humidity detection element, PA1 correcting the electromotive force with a linear relational expression of the electromotive force and the humidity, and PA1 determining a carbon dioxide concentration on the basis of the corrected electromotive force. PA1 measuring an electromotive force with the carbon dioxide sensor and at the same time detecting a temperature with the temperature detection element and detecting a humidity with the humidity detection element, PA1 correcting the electromotive force with a linear relational expression of the electromotive force and the temperature and a linear relational expression of the electromotive force and the humidity, and PA1 determining a carbon dioxide concentration on the basis of the corrected electromotive force.
In the interface of the CO.sub.2 detection electrode of the above cell, an equilibrium of EQU Na.sub.2 CO.sub.3 {character pullout}2Na.sup.+ +CO.sub.2 +1/2O.sub.2 +2e.sup.-
is maintained, and in the interface of the non-sensitive electrode, an equilibrium of EQU 2Na++1/2O.sub.2 +2e.sup.- {character pullout}Na.sub.2 O(NASICON)
is maintained. Therefore, the total cell reaction is described below. EQU Na.sub.2 CO.sub.3 {character pullout}Na.sub.2 O+CO.sub.2
The electromotive force E of the above cell is represented by the expression, EQU E=E.sub.0 -(RT/2F)1n(aNa.sub.2 O PCO.sub.2)
If 2Na.sub.2 O can be taken as constant, and when or if T can be taken as constant, the partial pressure of carbon dioxide can be determined on the basis of the electromotive force of the above cell. The above explanation is given in the proposal of Maruyama et al in "No. 10 Solid Ionics Symposium Lecture Summary Prints 69 (1983)".
In reality, however, Na.sub.2 O cannot be taken as constant in many cases, which causes the partial pressure of carbon dioxide determined on the basis of the electromotive force to vary, as has been pointed out.
Meanwhile, there has been proposed a solid reference electrode type carbon dioxide sensor obtained by attaching, under force, a solid reference electrode which is an electron and oxygen ion conductor to a solid electrolyte formed of an alkali ion conductor (JP-A-7-63726). In this Publication, an Li ion conductor is used as a solid electrolyte, and Li is incorporated into the solid reference electrode so as to keep the activity of LiO.sub.2 generated in the interface constant. Since, however, the above sensor also uses an alkali ion conductor, the operation temperature is as high as 400 to 500.degree. C. as shown in Examples in the Publication.
Many electromotive force detection sensors have detection electrodes formed of sodium carbonate, and the humidity resistance thereof is a problem due to their hygroscopicity, while many have been proposed to decrease the influence of humidity. For example, there has been proposed a material which is a solid solution of an alkaline earth metal carbonate and an alkali metal carbonate and does not contain a crystal of the alkali metal carbonate, as a coating material for the detection electrode (JP-A-5-80021, J. Electrochem. Soc., Vol. 130, No. 5, 1384, May 1992).
These electromotive force detection sensors require no reference gas, and reference electrode sides are in an atmosphere to be measured. Measurement values are therefore affected by an oxygen partial pressure. It has been therefore proposed to exclude an influence of a change in an oxygen partial pressure by tightly attaching and laminating an oxide ion conductor on the reference electrode side of an alkali ion conductor (JP-A-7-94013). Further, there are a growing number of examples having a structure in which portions other than the detection electrode are closed with a material impermeable to gases so that the detection electrode alone is in contact with an atmosphere to be measured. As the result of these improvements, stabilized measurement has come to be possible without being much affected by a humidity and an oxygen partial pressure.
For attaining a high conductivity with a solid electrolyte such as NASICON or .beta.-alumina (NA.sub.2 11AL.sub.2 O3) in an electromotive force detection sensor, however, it is required to provide a built-in heater and secure a high temperature of about 500.degree. C. like the above concentration polarization sensor. Further, an alkali metal carbonate or a solid solution of it with an alkaline earth metal carbonate, used as a detection electrode material, shows not only a low electron conductivity but also a low ion conductivity at room temperature, and a high temperature is therefore required to secure a response speed. A heat of several hundred degrees causes a delicate influence on a measurement environment since it heats vicinities of a sensor even if it is from a small heater, and it causes the convection of air. Further, the power consumption increases, and actuation with an electric cell is therefore difficult.
One report says that the operation of an electromotive force detection sensor at room temperature is attained by using a solid electrolyte prepared by replacing Na ion with Ag ion in NASICON (Bulletin of Niihama Technical Junior College (Science and Technology Book), 26, 98 (1990). Since, however, heating at several hundred degree is required in the case of Na ion, it cannot be thought that the ion conductivity is so improved by only substituting Ag ion. Specifically, the electromotive force at room temperature cannot be said to be sufficient. Further, it is also thought that the responsiveness has a problem.
As a solid electrolyte having a high ion conductivity at room temperature, a Cu ion conductor or an Ag ion conductor is known. Among these are, for example, solid ceramic materials such as RbAg.sub.4 I.sub.5 (2.7.times.10.sup.-1 Scm.sup.-1, 25.degree. C.), 75AgI.25Ag.sub.2 SeO.sub.4 (2.2.times.10.sup.-2 Scm.sup.-1, 20.degree. C.) and Ag.sub.3 SI (1.times.10.sup.-2 Scm.sup.-1, 25.degree. C.), and these can be also used to constitute sensors that can be operated at room temperature.
However, these solid ceramic materials need sintering around 1,000.degree. C., and they are generally poor in humidity resistance. In particular, the Rb-containing material is poor in humidity resistance and fragile. Therefore, the method of producing a sensor is limited.
In recent years, further, for use in Li ion cells, studies have been made on an electrolyte obtained by dissolving lithium perchlorate in a polymer such as polyethylene glycol or a gel type electrolyte formed from a crosslinked polymer and a lithium salt dissolved in a solvent. The later is actively studied in particular, so that a relatively high conductivity at room temperature is achieved. A sensor operable at room temperature can be obtained by combining the above gel electrolyte and electrodes, while the above gel electrolyte has highly hygroscopic properties. Differing from a cell having an electrolyte closed, the sensor has an electrolyte exposed to ambient atmosphere, and both the metal salt and the polymer are therefore liable to absorb water, to cause a change in electric conductivity, so that a stable electromotive force can be no longer attained.